A conventional light-duty vehicle engine is typically started at the beginning of a trip and remains active through the duration or length of the trip. The engine starting event draws a significant amount of electrical energy or power over a relatively short period of time ranging from approximately 0.3 seconds for a warm engine, to over 2 seconds for a very cold engine. Generally, a direct current (DC) electric motor powered by a single 12-volt battery is used to start the engine. Such a motor draws maximum amount of electrical current at its stall speed, with the electrical current decreasing as the speed of the motor increases. A typical 12-volt battery provides maximum current and minimum voltage during the initial portion of the starting event.
Before the engine of a conventional vehicle is started, the auxiliary electrical loads in the vehicle are powered by the 12-volt battery. Thus, all of the auxiliary loads experience a reduced voltage supply during the initial portion of the engine starting event. In some cases, this transient voltage reduction or voltage sag can cause a potentially noticeable or perceptible change in the performance of the auxiliary loads, such as a decrease in light intensity from incandescent lighting. Once the engine starts, the engine-driven generator produces the necessary electrical power for energizing the auxiliary loads, and may also recharge the 12-volt battery.
One method of reducing fuel consumption in a conventional vehicle is to shut off the fuel supply to the engine whenever the engine is not needed for supplying propulsive power. However, this method requires repeated engine restarts during a given trip, such as each time the vehicle is stationary at a stop light between end points of the trip. Additionally, the power delivered by the engine-driven generator to the auxiliary loads will be reduced to zero as the engine is shut off. The power required for powering the auxiliary loads is supplied by the 12-volt battery or by another power source if the vehicle is so equipped.
In some vehicles, the electrical power needed for restarting the engine is provided by a second battery which powers an alternative starting motor. For example, a belted alternator/starter utilizes a combined starter/generator as a secondary engine starting device in place of a conventional engine-driven generator. By using separate batteries to provide electrical power for the auxiliary loads and the secondary engine starting device, the electrical voltage supplied to the auxiliary loads is generally unaffected during the engine starting event, generally minimizing any customer-perceptible changes in performance of the auxiliary system. However, duplicate batteries contribute significant weight while consuming valuable packaging space within the vehicle.
In other light-duty systems, a single vehicle battery and starting device are always used for starting the engine. The generator is unchanged from that of the conventional vehicle. An electronic device known as a DC-DC converter takes electrical power supplied by the battery, regardless of the battery voltage, and produces a stable DC output voltage that is supplied to specific auxiliary loads, which could otherwise exhibit a change in performance during a transient voltage fluctuation during the engine starting event.